The polymer industry is constantly seeking to improve resin properties while maintaining or increasing polymer production. However, to avoid the risk of unplanned reactor shutdown, commercial reactors are typically operated at less than maximum production rates.
The production of off-grade polymer, that is, polymer not having the desired product properties, is due in large part to fluctuations or excursions in bed temperature during regular commercial operations. If the variation in the temperature of the fluidized bed during polymerization is too large, an unplanned reactor shutdown can result. Indeed, most unplanned reactor shutdowns are usually due to a variation in one or more operating constraints caused at least in part by inadequate temperature control of the fluidized bed.
Commercially, bed temperature control is accomplished by removing heat from the fluidizing or cycle gas via one or more water cooled heat exchangers. In this heat exchange system, the water flow is manipulated to remove heat from the cycle gas as the polymerization progresses. Typically, the water flow rate is increased in response to a rise in temperature of the cycle gas or the water flow rate is lowered in response to a decrease in temperature of the cycle gas. Generally, during polymerization there is a direct correlation of the cycle gas temperature and the temperature of the fluidized bed.
Temperature control for commercial operation as presently practiced is set forth schematically in FIG. 1. In this reactor system, warm cycle gas leaves the top of the reactor, passes through a compressor to a cooling tower in a heat exchanger, and, thence, cooled cycle gas returns to the bottom of the reactor. In FIG. 1, heat is removed from the cycle gas via a water cooled heat exchanger. The water flow to the heat exchanger is manipulated to remove heat from the cycle gas by adjusting the water flow valve (9) in response to temperature excursions as monitored by a temperature controller (4). That is, historically, bed temperature control is achieved through manipulation of the cycle gas heat alone. Heat removal from the cycle gas stream in this manner is a relatively slow process allowing significant variation in the bed temperature to occur. In the conventional system, water flow to the heat exchange system was manipulated to remove the cycle gas heat, while the cycle gas flow to and from the fluidized bed of the reactor is kept at a desired fixed (i.e., constant) value for any given polymerization process. Often, the cycle gas flow control element is kept fixed which in turn approximately fixes the gas cycle flow. In this bed temperature control configuration, bed temperature control was limited (i.e., provided a sluggish response) due to the slow dynamics of the water cooling system.
Accordingly, there is an on-going need for improved bed temperature control to provide improved control of product properties while maintaining or increasing production rates.